Erwinia bacteria are flagellated and rod-shaped. They are capable of surviving and thriving with or without oxygen.
These micro-organisms overwinter on infected plants, on the margins of cankers formed in the previous year and possibly in buds and branches over 1 cm in diameter.
The following spring, during warm, humid spells, the bacteria become active and multiply very rapidly: cell division may occur every thirty minutes. Masses of bacteria form, swelling tissues under the bark, which cracks and oozes a sticky liquid containing sap and bacteria. This sweet ooze attracts insects, which transfer the bacteria to other plants or other parts of the same plant.
The bacteria enter plants through flowers, wounds and even the smallest openings in plant tissue. Once inside, they move through the vascular system, rapidly blocking vessels and causing young shoots to wilt in the characteristic shepherd's crook.
During the growing system, the bacteria can also be spread by wind, rain, watering and handling by gardeners.
In late summer, the bacteria gradually become inactive and remain dormant until the following spring.